A Dependence of the ( pi +, pi + pi +/-) Reaction near the 2m pi Threshold.

The ${}^{2}\mathrm{H}{,}^{12}\mathrm{C}{,}^{40}\mathrm{Ca}{,}^{208}\mathrm{Pb}({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}})$ reactions have been studied at ${T}_{{\ensuremath{\pi}}^{+}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}282.7\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$, and the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ invariant mass distributions ${M}_{{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}}^{A}$ measured down to the ${2m}_{\ensuremath{\pi}}$ threshold. The ${M}_{{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}}^{A}$ yield near threshold is close to zero for $A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2,$ and increases dramatically with increasing A (from 12 to 40 to 208). A phase analysis indicates that the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ pairs in this range of ${M}_{{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}}^{A}$ have $J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}I\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$. In the case of ${M}_{{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{+}}^{A},$ there is some strength near threshold regardless of A, and the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{+}$ pairs have $J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0,I\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$. The experimental results indicate that nuclear matter strongly modifies the \ensuremath{\pi}\ensuremath{\pi} interaction in the $J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}I\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ channel.